The detoxification of ammonia in humans occurs primarily in the liver through the combined action of five enzymes. These enzymes catalyze the transformation of ammonia into urea for eventual disposition via the urine. The primary objective of this proposal is to elucidate the detailed mechanisms of action for argininosuccinate lyase and argininosuccinate synthetase. These enzymes catalyze the reactions that are currently thought to control the rate limiting steps in the metabolism of ammonia into urea. This research should have a significant impact in revealing the factors that control arginine and urea biosynthesis. This information is vital for the effective treatment of urea cycle diseases such as citrullinemia and argininosuccinate aciduria. Steady-state and rapid reaction kinetic studies along with magnetic resonance techniques will be the principle methods used in accomplishing these objectives. The primary aims of the proposed research are as follows. 1. Determine the identity of any intermediates involved in the reactions catalyzed by argininosuccinate synthetase and argininosuccinate lyase. 2. Quantitate all of the rate constants leading to the formation and breakdown of enzyme complexes occurring along the reaction pathways. 3. Identify the structural and functional roles of the divalent cations required in the argininosuccinate synthetase reaction. 4. Elucidate the amino acids at the active sites of these enzymes and determine the function of these groups in catalysis and/or binding of substrates and products.